Maintaining a sufficient compression pressure in a cylinder's combustion chamber internal combustion engine is vital for insuring proper combustion of air and fuel ingested into the cylinder. Typical causes of a low compression pressure condition in a cylinder may include damaged piston rings, valves, or head gasket to identify a few. Low compression pressure causes loss of power, misfires, incomplete combustion--resulting in increased hydrocarbon emissions, loss of fuel economy, and hard starts in diesel engines.
As the demand for increased engine performance increases, low cylinder compression pressure must be diagnosed in real time on an engine in service, so that the condition can be corrected as soon as possible.
For most contemporary engines compression pressure is checked by a service tester in a service environment. This tester is not indigenous to the engine. The engine is typically taken out of normal operating service so a compression pressure test may be run. These testers ordinarily rely on the connection of an in-cylinder pressure tester to a particular cylinder of the engine. The problems with this scheme include that the engine must be removed from normal operating service, compression pressure of only one cylinder can be measured at a time, the compression pressure is measured in a controlled environment and not as the engine is operating under actual conditions.
Another scheme, that measures in-cylinder combustion while the engine is operating in normal service, relies on individual sensors in each cylinder. These sensors are cost prohibitive, need to be individually calibrated, and also have limited durability. Also, individual mounting of sensors on each of the engine's cylinders is not desirable from a manufacturing viewpoint. Further, these sensors require space on the engine's cylinder heads that is scarce particularly in 4 valve engines, and interfere with the combustion process. Additionally, each sensor must be individually calibrated to eliminate internal errors.
Other prior art systems measure average engine crankshaft velocity to predict compression. Still other prior art schemes measure a time between two distal positions of the engine crankshaft to predict compression. These schemes are susceptible to inaccuracies associated with transient engine operation. Since an engine crankshaft can accelerate/decelerate at several thousand revolutions /sec.sup.2 any transient accelerations or decelerations occurring during the measurement of these velocities or times will cause the compression prediction to be inaccurate. For instance a deceleration may be observed as a loss of compression. Significantly, inaccuracy associated with transient engine operation can easily exceed the span of the measurement. For example, this may result in an indication of near zero compression when the cylinder is operating properly.
Also, prior art schemes have not accounted for the large difference in intake manifold pressure over the engine operating range. The intake manifold pressure, is ordinarily in the range of 30 kPa to 100 kPa absolute over the engine s normal operating range. This range is dependent on engine speed and load, and may vary another .+-.10% due to altitude changes. Because prior art schemes do not account for this variation in intake manifold pressure they can be in excess of 30% inaccurate.
Also, these prior art schemes fail to eliminate error due to non-combustion related torque influencing the compression measurement. These non-combustion related effects, which are typically attributable to variations in engine load torque, friction torque, and inertia torque, can add even more significant error to the prior art measurement schemes. Significantly, inaccuracy associated with non-combustion related effects can easily exceed the span of the measurement.
Since it is required to measure compression within a few percent in modern engines, these prior art schemes are grossly inadequate.
What is needed is an improved system for detecting low compression pressure in internal combustion engines that is more durable, cost effective, manufacturable, requires less calibration, and does not interfere with the combustion process.